ABSTRACT Mitochondrial dysfunction, oxidative stress, impairment of proteasomal function and protein aggregation are the common molecular basis of the loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpC). Mitochondrial dysfunction and increased vulnerability of dopaminergic (DA) neurons to oxidative stress has specifically implicated in the pathogenesis of PD. Molecular sources for reactive oxygen species (ROS) in PD however have not been clearly elucidated. ROS are physiologic byproduct of several organelles and biological reactions including mitochondria, peroxisomes, cytochrome P-450, xanthine oxidase, cyclooxygenase and NO synthase. A family of NADPH oxidase (NOX) is the first enzyme complex discovered which is specialized to generate superoxide. Nox homologues have been specifically identified in the central nervous system (CNS) and shown to play major roles in development, memory, neuronal signaling, and cardiovascular homeostasis. At the same time, an equally significant body of evidence has shown that overproduction of ROS by abnormal Nox activation may also contribute to neurodegeneration. Moreover, recent studies indicate that mitochondria play a role in NADPH oxidase-mediated (especially NOX1) superoxide generation. The interplay between mitochondria and NOX1 amplifies ROS generation and results in cell death. This suggests that two cardinal factors, mitochondrial dysfunction and oxidative stress, implicated in the PD pathogenesis may link each other through the NOX system. Our preliminary results demonstrate that DA cells are equipped with the NOX-mediated superoxide generation system. NOX1 was induced by oxidative stress such as 6-OHDA and mitochondrial toxin, rotenone. Inhibition of Rac1, a key component for Nox activation or Nox1 knockdown led to protection of substantia nigra DA neurons from 6-OHDA administration. LRRK2 mutation (G2019S) increased ROS generation in N27 DA cells and apocynin, a specific NADPH oxidase inhibitor reduced ROS production elicited by LRRK2 mutant. These proposed studies will investigate 1) whether inhibition of NOX1 leads to DA neuroprotection, 2) the molecular mechanism underlying DA cell-specific transcriptional regulation of NOX1 and mitochondrial involvement and 3) whether LRRK2 mutations affect the activation of NOX- mediated ROS production and consequential DA neurodegeneration.